The present invention relates to pattern defect inspection and foreign material inspection to detect such defects of a circuit pattern of a sample as shortage, breaking of wire and foreign matters. The invention particularly relates to a method and an apparatus for inspecting defects and foreign matters in such circuit pattern as of a semiconductor wafer, a liquid display, a photomask. In the description below, defects shall include foreign matters.
Conventionally, as this type of inspection apparatus, there is one that picks up an image of a sample using an imaging device, such as an image sensor, while moving the sample, and compares a detected image signal and that signal delayed by a fixed time, and thereby perceives a part in which the two image signals are not in agreement as a defect.
As another technology related to defect inspection of a sample, there is a technology of performing accurate inspection in a semiconductor wafer where an area of high pattern density, such as a memory mat part, and an area of low pattern density, such as a peripheral circuit, exist together in the same die. This technology is one that performs gray-scale translation on a digital image signal obtained by A/D converting a detected signal so that brightness values or contrast values of the high-density area and the low-density area of the pattern under inspection assume a predetermined relation, and compares the image signal whose gray scale was translated and an image signal for comparison whose gray scale was translated with physical positions of the two signals aligned.
As a technology of inspecting a circuit pattern of a photomask, there is a technology in which UV (UltraViolet) laser light is used as a light source, a mask is uniformly illuminated with light whose coherence is reduced by rotating a diffuser plate that is inserted in its optical path, feature quantities are calculated from obtained image data of the mask, and thereby the quality of the mask is determined.
As conventional technologies related to this invention, there are ones that are disclosed in JP 318326-H07A, JP 320294-H08A, JP 78668-H10A, JP 212708-S61A, JP 155099-2000A, and JP 194323-2001A, etc.
In LSI manufacture in recent years, circuit patterns formed on wafers have pattern widths equal to or less than 200 nm because of pattern miniaturization that corresponds to the need of higher integration, and accordingly dimensions of defects to be detected becomes finer. Especially, there is also reported a defect, called “Non Visual Defect,” that is hard to detect by the conventional technology. Under these circumstances, for defect detection apparatuses, development aiming at an objective lens for inspection with a higher NA (Numerical Aperture) and a super resolution technology has been advanced. However, since the higher NA of the objective lens for inspection has reached its physical limit, it is an essential approach to shorten the wavelengths of illumination light used for inspection toward regions of UV light and DUV (Deep UltraViolet) light.
Light sources of short wavelengths may include the UV light, the DUV light, VUV (Vacuum UltraViolet) light etc. However, under the present circumstances, they are generated mainly by pulsed lasers. However, the pulsed lasers have high peak powers, so it is very likely that a sample irradiated by that laser light might be damaged. Therefore, it can be considered that a laser with a reduced power level is used. However, in this case, it is necessary to lengthen storage time of an image sensor for detecting reflected light because of insufficient amount of light irradiated on the sample, and hence this measure comes with a problem of incompatibility with high-speed inspection.
When high-speed inspection is intended, an illumination method of scanning a narrowly converged laser beam on a sample is not suitable. Conversely, if a laser beam that is expanded as large as the whole visual field is used for illumination, there occurs speckle by the laser and signals of overshoot and undershoot, called ringing, are generated in edge parts of a circuit pattern; therefore, it is difficult to obtain a high-quality image, and otherwise it is necessary to reduce interference by the laser light.
Moreover, in the LSI device, structures of patterns to be inspected have become complicated and diversified, such as memory products formed mainly with a repeated pattern and logic products formed mainly with a non-repeated pattern, so it is difficult to find targeted defects surely. Furthermore, in the process of metal wiring, such as of Al (aluminum), minute irregularities called grain, pit, and morphology occur. Because this grain is detected as an optical noise during inspection, there is a problem that detected grain must be reduced. Moreover, in any process in which a transparent film (here, a term transparent is used to mean “being transparent at illumination wavelength”), such as an insulator film, is exposed as an uppermost layer, interference-light intensity unevenness arising from a minute difference in thickness of the transparent film causes optical noise. Therefore, there is a problem that the optical noise must be reduced.